Oil and gas wells have a finite productive life. Once all economically recoverable product has been removed from the well, the well is abandoned. Typically such abandonment involves cutting off the well casing below the ground surface, sealing the well, and then covering the sealed well with soil, effectively burying the top of the well.
A typical well will have a production casing extending down from the surface to the production zone where oil or gas is present. At the surface the production casing is located inside a surface casing that has a diameter larger than the production casing. The surface casing extends downward from the surface into the ground to protect the upper portion of the production casing.
During abandonment the surface casing will be cut down to a level below ground level, and the production casing will be cut down to a level below the top of the surface casing. A steel production plate will be fitted inside the top of the production casing and welded to the production casing to seal the casing. A surface plate will be fitted inside the top of the surface casing above the production plate and welded in place to seal the surface casing.
It is at times required or desirable to re-enter abandoned wells to inspect same or sometimes to re-work the well when the economics of recovery change, or when new technologies become available that may make it economical to operate the well again. Such re-entry can be hazardous as there is no provision to allow an operator to determine whether pressure has built up inside one or both of the casings, and so the sealing plates must be drilled to determine what is present in the sealed casings. The drilling tool must be housed inside a pressure control apparatus to control any pressurized gases that may be present under the plates. The shank of a drill bit will extend through a packing or like sealing element in the pressure control apparatus such that when the plate is pierced any pressurized gases that may be present are contained. Channels and caps on the pressure control apparatus allow the gases to be directed to a vent or containment vessel.
To re-enter a well it is typically required to weld the pressure control apparatus on to the surface plate. It is generally not possible or practical to determine the extent of any corrosion or like weaknesses that might be present in the casings or sealing plates, and the intense heat of welding may rupture same and cause a fire or other injury.